app, provision: provisioner should handle bind and unbind

[fsouza]

In order to get the proper origin hosts, we need to call the provisioner. In the case of Docker, all nodes that can be used for an app will be bound to the service instance, and whenever a new host is added to the cluster there will be a bind call to the service API.

In the Docker provisioner, these are the cases we're going to handle:

Binding an application to a service instance

Grab all hosts where units of the application can be born in, and make calls to the bind endpoint with the proper parameters app-host and unit-host.

Unbinding the application from the service instance

Grab all hosts where units of the application can be born in, and make calls to the unbind endpoint with the proper parameters app-host and unit-host.

Adding a new Docker node to the cluster

Grab all binds of all applications that will have access to the new node and issue proper bind calls to the service API (one call for each bind).

Removing a Docker node from the application cluster

Grab all binds of all applications that have access to the dying node and issue proper unbind calls to the service API (one call for each unbind).